In the case that a fuel cell utilizes pure hydrogen (hereinafter referred to as hydrogen) as a power source of an electric vehicle (hereinafter referred to as a vehicle), hydrogen is supplied to the fuel cell in a hydrogen circulation system in order to improve the utilization efficiency (fuel economy). In this circulation system, an ejector for absorbing hydrogen while generating negative pressure, or a hydrogen pump is employed. Both ejector and hydrogen pump require purging. Herein, the term “purging” means to temporally discharge hydrogen in the circulation system to the outside.
Purging is required for the following reasons:    (1) To recover decreased cell voltage during a travel of the vehicle or during a stop of the vehicle with the motor idling.    (2) To adapt the anode to the cathode in accordance with a pressure change of the cathode, so that when the vehicle is stopped, an excessively increasing pressure difference between the cathode and the anode of the fuel cell can be prevented.
The purposes for recovering the cell voltage are as follows:    (1) With continuously circulating hydrogen, N2 migrating from the cathode accumulates in the anode system, inhibiting the reaction. Therefore, it is necessary to discharge the accumulated N2.    (2) Since condensed dewdrops of supplied water or produced water pools and causes flooding within the fuel cell, the output of the fuel cell lowers. Therefore, it is necessary to discharge the dewdrops to the outside of the system by increasing the flow velocity of the gas by way of purging.
In the conventional fuel cell system where hydrogen is used as a fuel for the fuel cell, high concentrations of hydrogen is released from purge piping. In another example, hydrogen is released after combustion processing with the use of a catalyst.
However, if there is an open fire or naked flame on a road, an instantaneous combustion such as after burn occurs just after releasing high concentrations of hydrogen from the vehicle. This is not preferable in terms of appearance.
As shown in FIG. 9, the combustion characteristics of hydrogen when applying purging are such that hydrogen becomes highly combustible beyond the hydrogen concentration of 4% (percent by volume, hereinafter just referred to as %) and in a detonation region beyond the hydrogen concentration of 18.3% instantaneous and explosive combustion occurs.
When hydrogen is catalytically processed, purging is carried out intermittently at a predetermined interval as illustrated in FIG. 10. Therefore, it is very difficult to retain the catalyst at sufficiently high temperatures and to mix with air, which makes the whole system complicated.
Accordingly, in view of the fact that purging is carried out intermittently, the purpose of the present invention is to dilute hydrogen below the lower combustible concentration limit of the fuel and exhaust it by taking as much time as possible between purging of the fuel within the fuel cell system and the subsequent purging.